Optical band gap width in GaAs in megagauss magnetic fields

The band gap width in GaAs in magnetic fields of up to 10 MG is calculated using a five-band kp model. The selection rules for interband electron transitions in strong magnetic fields are found, and the dependences of the interband transition probabilities on a magnetic field are calculated. The electronic spectra calculated in the five-band model are compared with those calculated in the Kane model and in the tight-coupling approximation. The calculations are shown to agree with experimental data if the contribution from the density-of-states tails and excitonic effects to light absorption is taken into account.     

翠绿亚胺聚合物高压下芳环扭角和能隙的缩减

利用改进的Ginder-Epstein模型计算了翠绿亚胺聚合物在参数V4,0取值于1.8～35.8 eV的自洽变分基态,并通过芳环扭角的变化来估算聚合物压强,给出了V4,0,芳环扭角及能隙与理论压强之间的最小二乘拟合.结果表明随此参数的增大,芳环扭角和能隙都缩减而理论压强升高.当理论压强由零压增至3.0 GPa左右时,能隙先快后慢从2.0 eV减小至最小值0.87 eV.这理论与该聚合物的高压电导测试结果一致.     

翠绿亚胺聚合物高压下芳环扭角和能隙的缩减

利用改进的Ginder-Epstein模型计算了翠绿亚胺聚合物在参数V4,0取值于1.8—35.8 eV的自洽变分基态，并通过芳环扭角的变化来估算聚合物压强，给出了V4,0，芳环扭角及能隙与理论压强之间的最小二乘拟合。结果表明随此参数的增大，芳环扭角和能隙都缩减而理论压强升高。当理论压强由零压增至3.0 GPa左右时，能隙先快后慢从2.0eV减小至最小值0.87eV。这理论与该聚合物的高压电导测试结果一致。     

Enlargement of the band gap in the metal-insulator-metal heterowaveguide

We present a new metal-insulator-metal (MIM) heterowaveguide to enlarge the band gap, which is formed by alternately stacking two kinds of metals, modulating the MIM waveguide slit, and inserting different dielectric materials with the effective refractive index periodically modulated. Based on this structure, we adopt two different methods to enlarge the band gap: changing the thickness of the unit layer and combining two MIM structures. Both of them widen the band gap when surface plasmon polaritons propagate through the structure. This metal heterostructure is expected to have applications in surface plasmon polaritons (SPPs) based optical devices, such as filters, waveguides, especially for broad band gap elements.     

Local band gap modulation of AlGaAs grown on patterned GaAs substrates

The growth of AlGaAs layers on patterned GaAs substrates by molecular-beam epitaxy is strongly determined by the interfacet surface migration of Ga adatoms, leading to a pronounced variation of the local Al concentration. Taking advantage of this migration process, we have overgrown mesa structures, that consist of intersecting stripes or partially overlapping squares, to obtain a local maximum or minimum, respectively, of the AlGaAs band gap on top of the mesas.     

Electronic states near the band gap for the GaAs(110) surface

The recent theoretical controversy regarding empty surface states at the GaAs cleavage plane is examined by the exact solution of several tight binding models applied to a semi-infinite crystal. We correlate the existence of an empty surface gap state with the anion p/cation p character of this state. This work isolates the specific properties of a Hamiltonian for GaAs which yield band gap states on a relaxed surface and clarifies the essential differences among current theoretical treatments.     

Band gap and band offsets of GaNAsBi lattice matched to GaAs substrate

In this paper, we calculate the band gap and the band discontinuities of a GaN _y AsBi _x structure lattice matched to GaAs substrate using the conduction and the valence band anticrossing models at the same time. The results obtained show a good agreement with experiment. The nitrogen and the bismuth concentrations leading to a wavelength emission of 1.55 μm have been determined (x = 3.5%, y = 2%). This structure shows a good electron confinement resulting in a high characteristic temperature.     

The band gap and electrical resistivity of FeS2-pyrite at high pressures

The indirect energy gap and electrical resistivity of FeS₂-pyrite have been measured at high pressures and 300 K using optical absorption spectroscopy and electrical conductivity measurements. Absorption spectra extend to ∼28 GPa, while resistivity is determined to ∼34 GPa. The band gap of FeS₂ is indirect throughout this pressure range and decreases linearly with pressure at a rate of −1.13(9)×10⁻² eV/GPa. If this linear trend continues, FeS₂ is expected to metallize at a pressure of 80(±8) GPa. The logarithm of resistivity also linearly decreases with pressure to 14 GPa with a slope of −0.101(±0.001)/GPa. However, between 14 and 34 GPa, the logarithm of resistivity is nearly constant, with a slope of −0.011(±0.003)/GPa. The measured resistivity of pyrite may be generated predominantly by extrinsic effects.     

Direct determination of reduced band gap and chemical potential in an electron-hole plasma in high-purity GaAs

We report the first transmission experiments with tunable i.r. Laser light through an electron-hole plasma in high purity GaAs. Negative absorption (gain) is observed at energies below the chemical potential, and positive absorption above. The experimentally determined energetic positions of the reduced band gap and chemical potential are lower than theoretically expected.     

Efficient band gap prediction for solids

An efficient method for the prediction of fundamental band gaps in solids using density functional theory (DFT) is proposed. Generalizing the Delta self-consistent-field (ΔSCF) method to infinite solids, the Δ-sol method is based on total-energy differences and derived from dielectric screening properties of electrons. Using local and semilocal exchange-correlation functionals (local density and generalized gradient approximations), we demonstrate a 70% reduction of mean absolute errors compared to Kohn-Sham gaps on over 100?compounds with experimental gaps of 0.5-4?eV, at computational costs similar to typical DFT calculations.     

Transient saturation absorption spectroscopy excited near the band gap at high excitation carrier density in GaAs

Transient saturation absorption spectroscopy in GaAs thin films was investigated using femtosecond pump and supercontinuum probe technique at excitation densities higher than 1×10^{19}cm^{-3}. The Coulomb enhancement factor of the electron－hole plasma results in a spectrum hole at the pump wavelength. Two distinct transmission peaks at two sides of the pump wavelength are observed, arising from the bleaching of transitions from the heavy- and light-hole bands to the conduction band. The dynamic process of the transient saturation absorption is fitted using a bi-exponential function. The fast decay process is dominated by the carrier-phonon scattering and the slow process may be attributed to the electron－hole recombination.     

Photonic band gap architectures for holographic lithography

Using symmetry considerations, we identify three families of large photonic band-gap (PBG) architectures defined by the isointensity surfaces of four beam laser interference. For particular choices of beam intensities, directions, and polarizations, we obtain a diamondlike crystal, a novel body-centered cubic architecture, and a simple cubic structure with PBG to center frequency ratios of 25%, 21%, and 11%, respectively, when the isointensity surface defines a silicon (dielectric constant of 11.9) to air boundary.     

Effect of annealing on the surface and band gap alignment of CdZnS thin films

Effects of the annealing temperature on structural, optical and surface properties of chemically deposited cadmium zinc sulfide (CdZnS) films were investigated. X-ray diffraction (XRD) results showed that the grown CdZnS thin films formed were polycrystalline with hexagonal structure. Atomic force microscopy (AFM) studies showed that the surface roughness of the CdZnS thin films was about 60-400 nm. Grain sizes of the CdZnS thin films varied between 70 and 300 nm as a function of annealing temperature. The root mean square surface roughness of the selected area, particular point, average roughness profile, topographical area of roughness were measured using the reported AFM software. The band gaps of CdZnS thin films were determined from absorbance measurements in the visible range as 300 nm and 1100 nm, respectively, using Tauc theory.     

Observation of band gap and surface defects of ZnO nanoparticles synthesized via hydrothermal route at different reaction temperature

The effects of reaction temperature on the average particle size, surface defects and band gap of ZnO nanoparticles have been systematically investigated. The hydrothermal method was employed to synthesize ZnO nanostructures. The nanostructures of the resultant ZnO were studied by means of X-ray diffraction, Transmission Electron Microscopy, Ultraviolet-visible absorption, Raman, Fourier transform Infra-red and Photoluminescence spectroscopy. With increase in the reaction temperature, the peak position of the ultraviolet emission shifts slightly towards the red wavelength and the crystal quality was improved. The prepared ZnO nanoparticles have residual intermediate compound on the surface in the form of an acetate group, which acts as defect centers for the emission of yellow emission. Spectra analyses show that the visible emission depends strongly on the preparation conditions.     

Second indirect band gap in silicon

We report the first observation of the г₂₅→L₁ (second indirect) transition in Si based on optical absorption studies. The energy, (1650 ± 10) meV, measured for this critical point shows that there remains a large discrepancy between theoretical band structure calculations and experimental results for this material.     

Search target band gap with inverse band structure approach for complex superlattices

Because it is too difficult to pick out the needed structures from the vast possible configurations, complex superlattices have not been studied well for a long time. In this paper, an inverse band structure (IBS) approach which combined genetic algorithm search method with an empirical spds* tight-binding energy band calculation to address this problem is presented. Needed direct energy band gaps of Ga(Al)As complex superlattices are found by using this approach. It can be found that the band gap value can be same for different superlattice structures, but other properties of these complex superlattices could be different.     

Effect of anisotropy on the photonic band gap and surface polaritons of a one-dimensional single-negative photonic crystal

The effect of anisotropy on the photonic band structure and surface polaritons of a one-dimensional photonic crystal made of uniaxially anisotropic epsilon-negative (ε<0,μ>0) and mu-negative (ε>0,μ<0) metamaterials is theoretically investigated. Two different cases of uniaxially anisotropic epsilon-negative and mu-negative metamaterials are considered. It is found out that for one case of anisotropy, one-dimensional photonic crystal does not have any single-negative band gap. As a result, it can not support the surface polaritons. While, for another case, the structure shows single-negative band gaps. So, the surface polaritons can be excited at the interface of such a photonic crystal. However, these surface polaritons, unlike the isotropic case, are not omnidirectional and they are restricted to a limited rang of the propagation constant.     

Optical band gap of Si nanoclusters

We discuss the nature of the optical transitions in porous silicon and in Si nanoclusters in the light of recent theoretical calculations. The accuracy of the different techniques used to calculate the band gap of Si nanoclusters is analyzed. We calculate the electronic structure of crystallites in the Si-III (BC8) crystalline phase which is known to have a direct gap and we examine the effect of quantum confinement on clusters of SiGe alloy and amorphous silicon. The comparison with the experiments for all the systems suggests the possibility of different channels for the radiative recombination.     

Excitons in direct band gap cubic semiconductors

Perturbation is applied to study of the Wannier-Mott excitons in direct band gap cubic semiconductors with a fourfold degenerate highest valence band. The fine structure of exciton energy levels is investigated. General formulae are derived for the matrix elements of the perturbation. From these expressions it is straightforward to obtain values of the fine structure splittings of the energy levels and the wave functions of corresponding staes in any order of perturbation theory. A comparison with results of previous works is made.